1. Field of the Invention
This invention relates to a method for achieving the desired electrical conductivity in doped semiconductor materials that are affected by dopant passivation. The invention specifically teaches methods of fabricating passivation-barrier layers to prevent or reduce doping species passivation during the semiconductor growth process, thus eliminating the need for in-situ or ex-situ annealing steps.
2. Description of the Related Art
Semiconductor materials are essential for producing light emitting diodes (LEDs), laser diodes (LDs) and other optoelectronic and electronic devices because the electrical and optical properties of the material can be controlled through composition and structural variation. To achieve this control, it is important to produce semiconductor material free of undesired impurities. Gallium nitride (GaN) is one of the most promising semiconductor materials for application to blue, violet and ultraviolet (UV) LEDs and LDs as well as other electronic devices.
During growth of GaN p-n junction diodes using Metal-Organic Chemical Vapor Deposition (MOCVD), it is difficult to grow a p-type material with good structural, optical, and electrical integrity. The p-type region of a GaN junction diode is commonly grown in a MOCVD reactor by adding magnesium (Mg) to achieve the desired conductivity. However, a common problem is electrical passivation of acceptors like Mg, zinc (Zn), carbon (C) and others by hydrogen atoms. The smaller hydrogen atoms diffuse into the GaN material, where they neutralize the Mg acceptors and the holes produced by the Mg. The passivation process leaves the Mg acceptors inactive, resulting in the material becoming insulating or weakly p-type in its as grown state. The passivation of the p-type region results in critical performance problems for the diode. Hydrogen passivation of acceptors and donors has been reported for a wide variety of semiconductors including silicon [S. J. Pearton et. al., Appl. Phys. A 43, 153 (1987)], gallium arsenide [N. M. Johnson et. al., Phys. Rev. B 33, 1102 (1986); W. C. Dautremont-Smith, J. Appl. Phys. 66, 1993 (1989)], indium phosphide [G. R. Antell et. al., Appl. Phys. Lett., 53, 758 (1988)], and cadmium telluride [L. Svob et. al., J. Cryst. Growth 86, 815 (1988)]. Passivation has been demonstrated both intentionally and unintentionally as a result of the epitaxial growth process.
The growth of GaN diodes represents one example where hydrogen passivation plays an important role. It has been shown that passivation of acceptors occurs after growth, during the reactor cooling stage. [G. R. Antell et al., Appl. Phys. Lett. 73, 2953 (1998)]. Hydrogen is common in a MOCVD reactor during growth of the GaN material and subsequent reactor cooling, generally coming from two sources. Hydrogen is commonly used during growth as a carrier gas for the growth source gasses. In addition, ammonia (NH3) is used as a source gas for nitrogen (N) during growth of the GaN material and is also used to stabilize the GaN material during reactor cooling. Hydrogen is produced as a by-product of the ammonia decomposition during growth and cooling. In the conventional GaN growth process, there is sufficient hydrogen in the reactor to cause passivation of the p-type region during cooling.
Passivation of the p-type region could be avoided by removing the hydrogen source from the reactor prior to cool down. See U.S. Pat. No. 5,891,790, to Keller et. al. However, the GaN crystal is unstable at growth temperatures and the p-type GaN region is susceptible to decomposition which results in surface damage. The conventional method for avoiding this decomposition is to maintain the flow of NH3 during reactor cooling. However, the presence of NH3 during reactor cooling produces hydrogen and leads to passivation. As such, it was thought that the removal of all hydrogen sources was not practical and that passivation during reactor cooling could not be avoided.
As opposed to avoiding passivation, various methods were developed to reverse the passivation after material growth and reactor cooling. Much of the early studies of dopant passivation revealed the utility of thermal annealing for removal of the hydrogen and xe2x80x9cactivationxe2x80x9d of the dopants [G. R. Antell et. al., Appl. Phys. Lett., 53, 758 (1988); W. C. Dautremont-Smith, Mater. Res. Soc. Symp. Proc. 104, 313 (1988); T. Zundel and J. Weber, Phys. Rev. B 39, 13549 (1988); W. C. Dautremont-Smith et. al., J. Appl. Phys. 66, 1993 (1989)]. Once the phenomenon of hydrogen passivation was recognized, it became evident that the hydrogen could be diffused into and out of the semiconductor at elevated temperatures, resulting in passivation or activation of the dopants. Consequently, thermal annealing was utilized as a method for acceptor and donor activation in group IV, III-V, and II-VI semiconductor materials. Acceptor passivation in Gallium Nitride could also be reversed using Low-Energy Electron Beam Irradiation (LEEBI) [Amano et. al., Japanese Journal of Applied Physics, Vol. 28, No. 12, pp. L2112-L2114 (December 1989)]. In LEEBI treatment, the passivated GaN material is irradiated with an electron beam to activate the acceptors. While these procedures effectively activate the passivated region, they are conducted ex situ, introducing another step in the fabrication process that increases cost and reduces yield. Damage can be caused from handling, and impurities can be introduced by atmospheric conditions. Both processes expose the GaN material to high temperature, which can also damage the material.
In addition, the annealing process occurs prior to p-contact metalization. In devices such as GaN LEDs and LDs, it is important to provide reproducible low-resistance contacts. Metal contacts to p-type GaN have been found to be highly sensitive to surface quality. [J. Kim et. al. Appl. Phys. Lett. 73, 2953 (1998)]. Thus, any damage to the p-type surface during annealing or handling will adversely impact the p-type metal contacts.
Another method for reducing passivation of acceptors during reactor cooling is capping the p-type material with a thin n-type layer. [S. Manigawa and M. Kondo, J. Electron. Mater. Vol. 19 No. 6, pp. 597-599 (1990)]. The solubility of hydrogen in the n-type material is low compared to the p-type material, so diffusion of hydrogen during reactor cooling is greatly reduced. However, this process does not completely eliminate the passivation of the p-type material and removal of the n-type material by etching is difficult and can damage the p-type surface.
The present invention provides a novel method of achieving the desired electrical conductivity in doped semiconductor materials that experience passivation of the doping species by hydrogen atoms. Passivation occurs by hydrogen being incorporated into the semiconductor material in association with doping species.
The invention specifically teaches methods of fabricating barrier layers to prevent or reduce the diffusion and incorporation of hydrogen into the semiconductor material, thus preventing or reducing dopant passivation. The barrier layers are fabricated in the same growth chamber that is used for the semiconductor material growth. Shortly after the growth of the material, while it is in the reactor and the reactor is at a temperature about equal to or lower than the growth temperature, the diffusion barrier layer is deposited on the material. The barrier layer is formed from dense, inert compounds that block the diffusion and incorporation of hydrogen into the material. Examples for barrier layers are Si, Ge, MgOx, MgNx, ZnO, SiNx, SiOx, alloys or layer sequences thereof.
The barrier layer can also be a hydrogen binding layer, also referred to as a hydrogen-gettering layer. The hydrogen-gettering layer chemically binds hydrogen trapped in the semiconductor or prevents hydrogen from reaching the semiconductor surface from the ambient gas phase, thus preventing hydrogen diffusion into the semiconductor. The composition of the gettering layer compounds can be adjusted during growth to contain an excess of the gettering component. The preferred gettering layer materials contain components that have a high binding energy to atomic hydrogen. The binding energy is also preferably higher than the binding energy between hydrogen and the acceptor or donor species. Thus, the layer can be effective in preventing the passivation of doping species by gettering the hydrogen. The layer can have multiple layers that can be a repeated sequence of multiple gettering materials.
After cool-down, any of the barrier layers may be removed by one of a variety of etching techniques for subsequent processing of the semiconductor material.
Eliminating passivation of the semiconductor material also can eliminate or reduce the need for additional processing steps such as thermal or electron beam annealing. The method also provides superior quality and yield in semiconductor materials, preserving the material in its pristine as grown state.
These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: